The present invention relates to a high speed tunable microwave cavity. More specifically the present invention relates to a microwave cavity loaded with a ferrite which exhibits the magneto-electric effect and that has a voltage applied across the ferrite which determines the resonance frequency of the cavity.
Microwave beam shifting was originally produced by mechanical systems, or by a system of manifolding to distribute microwave energy to individual waveguide elements, where each element contains a phase shifter followed by an antenna transmitter. In the manifolded system, beam shifting is accomplished by applying electric currents to the phase shifters. The mechanical method of beam shifting is slow and readily subject to wear and malfunction. The manifolded method of beam shifting requires a considerable amount of space to accomplish power distribution and each consecutive phase shifter must shift a greater amount to maintain proper beam proportion, since phase shift is not additive or reciprocal. An improved version was devised in U.S. Pat. No. 3,069,680 with the cavities being tuned by changing a magnetic field. In U.S. Pat. No. 3,069,680, the beam shifting system used ferrite-loaded solenoid-wound cylindrical cavities coupled with a slot array waveguide. The use of ferrite-loaded phase shift cavities induces phase shift into a waveguide containing a slot array and causes the beam lean angle to change. The phase shift is varied by changing the D.C. current in the solenoid coil around the cavity. Since the permeability of the ferrite is varying with current drive, the phase shift of the microwave signal propagating in the waveguide is also varied. The microwave cavity is coupled electromagnetically to the waveguide. The tuning speed is limited by the time constant of the solenoid coil, which is given as EQU .tau.=L/R
where L and R are the inductance and resistance of the coil wrapped around the cavity. Typically, L=0.1 mh and R=1 ohm. This means that the tuning speed of the cavities used in U.S. Pat. No. 3,069,680 is of the order of 0.1 msec.
A problem associated with the use of magnetic materials in modulating microwave signals is that the magnetic properties of the materials change with temperature. In microwave devices where the ferromagnetic resonant frequency of a magnetic material is required to be stable, temperature fluctuations are inadvertently introduced into such systems. These temperatures fluctuations may give rise to undesireable modulation effects of a microwave signal.
Additionally, where the ferromagnetic resonance is required to be tuned, the tuning is achieved by changing a magnetic field provided by a magnetic field generating device, e.g., a helmholtz coil. Typically a helmholtz coil with a 1 ohm resistance and a 1 inch diameter requires about 10-100 .mu. watts to generate a given amount of tuning. If there is more than one ferrite device, the high wattage consumption is undesireable.